The aim of this project is to develop a new technology that can be used to detect very low concentrations (femtomolar and below) of proteins that are diagnostic for cancer. Presently, early diagnosis of cancer is limited by the fact that the limits of detections (LOD) of available technologies, such as ELISA, are higher than the circulating concentrations of low abundance proteins that could indicate the onset of disease. This program would broaden and strengthen the use a novel fiber optic array single molecule detection technology for extremely high sensitivity measurement of cancer markers. This technology is digital in nature, i.e., individual molecules are quantified by their presence ("on" signal) or their absence ("off" signal). By focusing detection at the single molecule level, dramatic improvements in sensitivity are achieved for proteins. Phase I of this project demonstrated that single molecule detection makes it possible to detect concentrations of proteins at levels a thousand times lower than ELISA. Using single molecule detection, prostate specific antigen (PSA) was detected in serum at concentrations less than 1 femtomolar;current immunoanalyzers have an LOD of about 3 picomolar for PSA. The Phase II project has three specific aims that would build on these proof-of-principal experiments to broaden and strengthen the use of the technique for detection of cancer protein markers, and to move the technology closer to commercialization for use by medical researchers and clinicians. First, the ultra-sensitive PSA assay will be used to detect the marker in human clinical samples, specifically to measure the levels of PSA in prostate cancer patients who have undergone radical prostatectomy. Current technologies cannot detect PSA in these samples;this technology could enable the early detection of the return of PSA in serum, which would indicate residual disease. Second, the technology will be expanded to cover more classes of proteins, such as fusion proteins and post-translational modifications, to enable its broader implementation. Third, methods will be developed to measure panels of proteins at low concentrations using small sample volumes. These panels would enable the diagnosis of difficulty to detect diseases, such as ovarian cancer, with greater sensitivity and specificity. The long term objective of this research is to establish a new standard in high sensitivity detection of biological molecules (proteins, peptides, RNA, cells) using instrumentation that would be widely employed in both academic and clinical laboratories. Such an innovation would enable researchers to detect cancer markers at unprecedented levels for early diagnostic applications, and would facilitate the discovery of new low abundance cancer markers. This long term goal satisfies several aspects of the mission of the NIH, in particular, to develop innovative research strategies towards improving the diagnosis, prevention and cure of human diseases. The early diagnosis of cancer is determined by the ability of the clinical oncologist to detect the disease. Quanterix is proposing to develop a technology that would enable clinicians to detect very low concentrations of proteins in blood and, therefore, enable the early diagnosis of disease. This technology has the potential to help medical researchers and doctors to identify and detect many more early cancer diagnostic markers and improve outcomes for patients.